A scumtell, strictly speaking, is an action that reveals a player to be scum. The term "tell" originates from card games such as poker, wherein players will subconsciously or involuntarily reveal how good their hand is and the other players can act accordingly.
In practice, "scumtell" is used to describe scummy actions; that is, actions that scum players are more likely to do than Town.
Scumtells are usually divided between game-based tells and meta-based tells. Game-based tells, or "objective scumtells", are generic tells that scum will generally fall into regardless of who they are. JEEP's Common Tells are examples of this - players would commit these tells as any alignment without realizing it, simply because it appeared to be advantageous for them to do so. Meta-based tells, or "personal scumtells", are tells that are specific to a player. For instance, an ordinarily active player who is known to lurk as scum can be said to have a meta-based scumtell - if they lurk, they're more likely to be scum.
It is worth noting that while "tells" are wonderful when other players commit them, players themselves do not like being read. After all, a scum player who can easily be read as scum is not a very good scum player. Thus, as soon as players become aware of their own meta, they will generally start trying to change them or intentionally break them.
Thus, tells that are published or become common knowledge tend to stop working fairly quickly after players start to catch on. JEEP's Common Tells were nothing short of miraculous in 2003, but after becoming ingrained in site culture, they are now historical artifacts more than anything.
See also Meta.